The alveoli of the lung of the animal are lined with a physiologically active substance which mainly comprises phospholipid and is called a pulmonary surfactant. While covering the inner walls of alveoli, this substance acts to protect the alveolar epithelium and has an important physiological function for the animal to maintain its respiratory function. More particularly, it is said that the pulmonary surfactant exerts a specific surface action to cause changes in the surface tension of the inner surfaces of alveoli in response to expiration and inspiration, thus contributing to the maintenance of stability among the alveoli and displaying the anti-atelectasis activity as well. The insufficiency of such a pulmonary surfactant invites the collapse of the alveoli making them impossible to keep the stabilized ventilation and thus causing the idiopathic respiratory distress syndrome (IRDS) which is sometimes seen with newborns.
Means can be adapted to prevent a newborn from being born with such a syndrome: when the result of the determination of the pulmonary surfactant content in the amniotic fluid, which is correlated to the growth of the fetal lungs, shows that the fetus is going to be born with immature lungs, it is possible to exercise such intrauterine therapy for the fetus as augmentation of secretion of the pulmonary surfactant by the administration of steroids.
As regards the methods of determining or estimating the content of pulmonary surfactant in the amniotic fluid, several methods have hitherto been proposed. For instance, there is a method in which the L/C ratio (ratio between lecithine and sphingomyelin) in the amniotic fluid is determined and another method in which the amount of dipalmitoyl phosphatidyl choline (DPPC) in the amniotic fluid is measured at the marker of the pulmonary surfactant; however, the former has a demerit of presenting a low correlation with IRDS since the method does not determine the phospholipid content which is the main component of the pulmonary surfactant and the latter involves a problem of having a low sensitivity. Incidentally, about 90% of the pulmonary surfactant is lipids such as phospholipid and neutral lipid and about 10% is protein. They exist as a complex of lipid and protein, i.e. as a lipoprotein. The removal of lipids from the pulmonary surfactant gives water insoluble protein, which is called apoprotein mainly composed of protein with molecular weight of about 36,000 (36K). Since protein excels phopholipid in specificity and can be detected at higher sensitivity, studies have been made to use the protein as the marker of the pulmonary surfactant and immunological quantitation by use of polyclonal antibodies has also been attempted. However, problems are also found with the method, in which polyclonal antibodies are used, in that the determination procedure takes a long time and that the sensitivity is not enough.
For carrying out the determination of a very small amount of the pulmonary surfactant contained in such a test substance as amniotic fluid in a short time with high sensitivity, it is necessary to obtain monoclonal antibodies that react against the pulmonary surfactant, thus solving all the abovementioned problems, but such monoclonal antibodies have not been obtained yet up to now.